This invention relates in general to pulsed RF systems and more particularly to a double detection receiver to be employed in such a system.
Pulsed RF systems have been widely employed and sometimes they are referred to as on/off keying systems (OOK) or amplitude shift keying systems (ASK). In such systems, a carrier amplitude is turned on and off generating a pulsed waveform which essentially consists of rectangular pulses. These systems may employ pulse shaping such as raised cosine, etc. or other types of shaping which are used to conserve bandwidth.
Essentially, such systems utilize relatively simple receivers wherein the detection can be either coherent or non-coherent. In any event, the systems also employ single and double detection schemes in the receivers. These receivers are used to receive and demodulate the pulsed RF signal by either scheme using crystal video detection with RF amplifiers and RF bandpass filters for selectivity.
Both single and double detection receiver systems were attendant with various problems. In the single detection receiver system, one required relatively expensive filters in order to effectively respond to the pulsed RF signal. Hence such systems employed complicated filter designs which required substantial tuning and adjustment.
The single detection receiver systems employed filters which had many poles and hence were attendant with group phase delay and other transmission delays which again resulted in serious problems. In regard to the single detection system spikes are also produced by a single channel receiver utilizing a multipole filter when the receiver was subject to strong short rise time RF pulses. Such pulses which are 20 db or more down the filter skirt would cause spikes to appear in the detected video signal. In any event, these spikes caused false thresholding and false outputs making the single detection receivers unreliable upon the receipt of certain signals.
As indicated above, while single detection receivers have been widely employed they are attendant with the problems in producing and fabricating complex filters using multiple poles thereby causing excessive amounts of phase distortion and filter delays. In order to circumvent and render the filtering problem simpler, the prior art then utilized double detection receiver systems. In any event, these receiver systems have never found acceptance because of other disadvantages.
In such receivers the threshold vs. frequency was not as constant from band edge to band edge as with a single detection receiver utilizing multipole Chebychev filters. The go/no go filter skirts did not have a "brickwall" response but approximated a nine-pole response. What is meant by a "brickwall" response is that essentially all frequencies outside the desired band, when encountering the filter section of the receiver, should encounter an extremely steep filter skirt to prevent those frequencies not within the desired band from passing through the receiver.
Hence for frequencies outside the desired receiver bandwidth, those frequencies should encounter a virtual brickwall in regard to filter response. The term "brickwall" is a term of the art which essentially means an extremely steep filter skirt which thereby prevents any frequencies not within the desired bandpass from passing through the receiver.
In regard to double detection receivers the group delay varied with input pulse rise time as well as with amplitude and RF frequency variation and hence made the previous double detection system impracticable in systems which required constant or predictable delay or phase. This was also, as indicated above, a problem with single detection receiver systems. Also the subtractor which is employed in such receivers produced a negative going video pulse when just outside the designed bandwidth. A negative going 2 percent duty cycle pulse appears like a 98 percent duty cycle pulse and the receiver operates to process it as such hence giving false results. In order to circumvent this problem, the prior art devised various clamping devices which were found to be in-effective.
Thus it is an object of the present invention to provide an improved double detection receiver which circumvents many of the problems as indicated above.
It is a further object of the present invention to provide a double detection receiver having an improved response allowing one to implement simple filter designs while circumventing many of the problems as indicated above.